Safety devices for motor vehicles, such as the airbag system, have components which are operated electrically. For reliable operation of these safety devices, a failure of the electrical components due to voltage variations in the electrical system of the vehicle must be avoided.
It is known to counter an overly great decrease in the voltage of the electrical system with a voltage transformer which forms a correspondingly greater supply voltage for the operation of the safety device from the low electrical system voltage still present. Furthermore, so-called energy reserves are used, which maintain the ability of the safety device to function for a short period of time, after failure of the voltage supply.
U.S. Pat. No. 4,384,734 describes a passenger safety device for a vehicle equipped with an electrical system. This has at least one electrical component and one energy reserve circuit for temporarily maintaining the voltage supply for triggering a safety device. If an excess voltage protection device is provided in this known device, as is usual, but not mentioned in the document, and if a maximum value of the electrical system voltage which can be predetermined is exceeded, the automatic result is that the safety device is disconnected from the electrical system voltage and connected with the energy reserve voltage.
Finally, a device for avoiding data loss in a temporary memory in the case of failure of an alternating current network is known from U.S. Pat. No. 4,381,457. This device comprises a monitoring device having a window comparator, which connects the device to an auxiliary alternating voltage source.
From U.S. Pat. No. 4,835,513, a voltage supply supported by capacitors is also known, which further has a comparator with a test connection for function testing.
An object of the invention deals with the problem of always keeping the electrical energy which is sufficient to operate or trigger the safety device available even in atypical operating conditions.